Electrical submersible pumps (ESP) are commonly used to pump well fluid from oil producing wells. A typical ESP has an electrical motor that drives a pump. The motor is filled with dielectric motor lubricant for lubricating motor bearings. A pressure equalizer or compensator couples to the motor for reducing a pressure differential between well fluid on the exterior of the motor and the motor lubricant. The pressure equalizer may be on the upper end or the lower end of the motor.
One type of pressure compensator has a metal bellows with a side wall having corrugations. Normally, an interior of the bellows contains motor lubricant that is in communication with motor lubricant in the motor. A well fluid port in the housing containing the bellows admits well fluid to the exterior of the bellows. The bellows extends and contracts in response to differences between the well fluid pressure and the motor lubricant pressure.
One bellows arrangement comprises a larger diameter or outer bellows joined with a smaller diameter or inner bellows. The outer bellows connects to a port at one end of the housing, and the inner bellows joins a port at the other end. The interiors of the inner and outer bellows are in fluid communication with each other. The inner bellows allows the length of the outer bellows to extend and contract. The inner and outer bellows arrangement works well but the combined arrangement is more costly than a single bellows.